Re: [PATCH v2 2/5] tsm: Introduce a shared ABI for attestation reports

[Greg Kroah-Hartman]

On Mon, Aug 14, 2023 at 09:43:37AM -0700, Dan Williams wrote:
> Greg Kroah-Hartman wrote:
> > On Mon, Aug 14, 2023 at 12:43:21AM -0700, Dan Williams wrote:
> > > One of the common operations of a TSM (Trusted Security Module) is to
> > > provide a way for a TVM (confidential computing guest execution
> > > environment) to take a measurement of its launch state, sign it and
> > > submit it to a verifying party. Upon successful attestation that
> > > verifies the integrity of the TVM additional secrets may be deployed.
> > > The concept is common across TSMs, but the implementations are
> > > unfortunately vendor specific. While the industry grapples with a common
> > > definition of this attestation format [1], Linux need not make this
> > > problem worse by defining a new ABI per TSM that wants to perform a
> > > similar operation. The current momentum has been to invent new ioctl-ABI
> > > per TSM per function which at best is an abdication of the kernel's
> > > responsibility to make common infrastructure concepts share common ABI.
> > > 
> > > The proposal, targeted to conceptually work with TDX, SEV, COVE if not
> > > more, is to define a sysfs interface to retrieve the TSM-specific blob.
> > > 
> > >     echo $hex_encoded_userdata_plus_nonce > /sys/class/tsm/tsm0/inhex
> > >     hexdump /sys/class/tsm/tsm0/outblob
> > 
> > Why is one way a hex-encode file, that the kernel has to parse, and the
> > other not?  Binary sysfs files should be "pass through" if at all
> > possible, why not make them both binary and not mess with hex at all?
> > That keeps the kernel simpler, and if userspace wants the hex format,
> > they can provide it much easier (with less potential parsing errors).
> 
> I can do that. The concern was the contract around what to do with
> partial writes since binary attributes allow writing the middle of the
> buffer. So either the attribute needs to enforce that @offset is always
> zero, or that the unwritten portion of the buffer is zeroed. I will go
> with just enforcing offset=zero writes.

Enforcing that sounds sane, thanks.

greg k-h